This invention relates to switched reluctance motors and, more particularly, to an improved magnetic sensor for use with such motors.
Ring magnets are used in various industrial applications for sensing purposes. One of these applications is in dynamoelectric machines such as electric motors. In use on such motors, for example on switched reluctance motors (SRM's), the magnetic sensor comprises a ring installed on the rotor shaft of the motor so it turns as the rotor turns. When used with a Hall effect sensor, the magnetic sensor provides positional information which is used by control electronics for the motor to determine motor speed. This information is usable, in turn, to control switching between motor phases for commutation purposes. Conventionally, the magnet ring has opposed poles with the north and south poles subtending equal arcs about the circumference of the ring. This means that for each motor revolution, the Hall sensor sees each pole an equal amount of time. With respect to application of current to these polyphase motors, the effect is that the turn-on and turn-off portions of each cycle are approximately equal. In some polyphase SRM applications, it may be desirable to operate the phases so that there is a disparity in the length of these portions of the cycle. For example, in some applications it may desirable for the turn-on time of a SRM motor phase to last only 30%-45% of the turn-off dwell time. Or, in other applications, it may be desirable for the turn-off time to only be 30%-45% of the turn-on time. From a control standpoint, there is a problem in achieving this type of control using the present ring and sensor capabilities which are available.